5 ms·

QFNs and most reflows are tricky as well, since it requires you to also design the board properly to reflow correctly. For example, if the QFN has a large groun

by tcas 13y ago

QFNs and most reflows are tricky as well, since it requires you to also design the board properly to reflow correctly. For example, if the QFN has a large ground pad in the middle, putting unmasked vias in the middle can cause solder to get sucked down to the other side. To the best of my knowledge, there isn't a great handbook that explains all this, since it crosses a bunch of fields (the electrical design says you will want a few vias in the pad to lower impedance, while the manufacturing might say otherwise [0] or will be able to do it no problem). In regards to memory and more advanced stuff, the beaglebone/raspberry pi/other system on modules are great for the hobbyist and low volume production, since making what is essentially a computer can be very difficult. It might work on the first spin, but hard to spot issues might occur if the power / grounding isn't good enough, or if there are noise or other signal integrity issues. [0] http://i.screamingcircuits.com/docs/Via_In_Pad_Guidelines.pdf http://i.screamingcircuits.com/docs/Via_In_Pad_Guidelines.pd...

EChrisDenney 13y ago
Hi tcas, Chris from Worthington Assembly here. We found that when we switched to our super fancy high end reflow oven (that's literally 15 feet long) all those sorts of issues kind of went away. Stencil design has a lot to do with it, believe it or not. Too much paste on the center pad and the QFN just floats up and the joints never get connected. So yeah, if CircuitHub just has a footprint for QFN's that's known to work, you can eliminate a lot of that type of research.
- tcas 13y ago
  I believe that. Even for 0.5mm QFPs, trying to do a stencil at home controlling the amount of solder paste is very difficult. Almost every attempt I've tried leads to at least a few bridges (easy to fix, but still annoying). With the professional machines you guys have it's probably much more reliable even with not so great stencils. Do you end up doing a final DFM as well for most submitted stencil files?
  - EChrisDenney 13y ago
    Yeah, every stencil file gets thoroughly reviewed. We reduce aperture sizes for fine pitch components, "window pane" large apertures, round-rects on BGA's, all that kind of stuff. If you choose a CircuitHub approved footprint, all that kind of stuff will be taken care of automatically. Using high-end machines makes life a lot easier too because we have a controlled snap off speed and angle as well as consistent squeegee pressure and paste bead height. You wouldn't believe how many factors go into getting a good stencil print.
lgeek 13y ago
> QFNs and most reflows are tricky as well I've now manually soldered two dozen or so 0.5mm pitch QFN chips without a single failure. To be honest, I'm somewhat surprised myself. I'm using the footprint recommended by the manufacturer, NXP, which has small vias on the edges of the center pad. Since the center pad is VSS, at least one via would be required anyway. My process is quite simple: 1) I use a soldering iron to tin all the pads except the large one in the center. 2) I apply no clean flux to the freshly tinned pads. If I wouldn't be too lazy to make a stencil I could probably skip these two steps. 3) I apply a blob of solder paste to the center pad. 4) I place the chip. 5) I reflow the whole board with a hot air soldering station. After the solder has melted, I tap the center of the chip to make sure all the pads on the chip have made contact with the pads on the board. I actually prefer QFNs over QFPs because on QFPs it's really easy to get solder bridges - easy to fix, but more finicky than preparing QFNs in the first place. I realise this doesn't really apply to manufacturing, but automated manufacturing already has the processes to deal with QFNs.
- cottonseed 13y ago
  Thanks! This is super useful. I had a QFN solder failure on one of my last boards. I mistakenly tented the center vias on the bottom, forgot to clear them before reflow, and it seems hot gas escaped, popping the chip off! But it left perfectly tinned pads. I will try your method out. You reflow the whole board with a hot air station? I've been using an oven. How big a board can you do that way?
  - lgeek 13y ago
    > You reflow the whole board with a hot air station? I've been using an oven. How big a board can you do that way? Yep. It's not really by choice, but because I have limited space. That being said, I like being able to watch the process and to correct minor issues like a 'tombstone' passive, so I think I'd prefer a hot plate over an oven. My SMD boards are small, the largest one by length is 7 cm and the largest one by area is 25 sq cm. You could definitely do larger ones, but you'd probably get significant temperature gradients - bad for manufacturing, probably ok for prototyping and one-offs.
  - 13y ago

by tcas 13y ago

EChrisDenney 13y ago

Hi tcas, Chris from Worthington Assembly here. We found that when we switched to our super fancy high end reflow oven (that's literally 15 feet long) all those sorts of issues kind of went away. Stencil design has a lot to do with it, believe it or not. Too much paste on the center pad and the QFN just floats up and the joints never get connected. So yeah, if CircuitHub just has a footprint for QFN's that's known to work, you can eliminate a lot of that type of research.

tcas 13y ago
I believe that. Even for 0.5mm QFPs, trying to do a stencil at home controlling the amount of solder paste is very difficult. Almost every attempt I've tried leads to at least a few bridges (easy to fix, but still annoying). With the professional machines you guys have it's probably much more reliable even with not so great stencils. Do you end up doing a final DFM as well for most submitted stencil files?
- EChrisDenney 13y ago
  Yeah, every stencil file gets thoroughly reviewed. We reduce aperture sizes for fine pitch components, "window pane" large apertures, round-rects on BGA's, all that kind of stuff. If you choose a CircuitHub approved footprint, all that kind of stuff will be taken care of automatically. Using high-end machines makes life a lot easier too because we have a controlled snap off speed and angle as well as consistent squeegee pressure and paste bead height. You wouldn't believe how many factors go into getting a good stencil print.

lgeek 13y ago

> QFNs and most reflows are tricky as well I've now manually soldered two dozen or so 0.5mm pitch QFN chips without a single failure. To be honest, I'm somewhat surprised myself. I'm using the footprint recommended by the manufacturer, NXP, which has small vias on the edges of the center pad. Since the center pad is VSS, at least one via would be required anyway. My process is quite simple: 1) I use a soldering iron to tin all the pads except the large one in the center. 2) I apply no clean flux to the freshly tinned pads. If I wouldn't be too lazy to make a stencil I could probably skip these two steps. 3) I apply a blob of solder paste to the center pad. 4) I place the chip. 5) I reflow the whole board with a hot air soldering station. After the solder has melted, I tap the center of the chip to make sure all the pads on the chip have made contact with the pads on the board. I actually prefer QFNs over QFPs because on QFPs it's really easy to get solder bridges - easy to fix, but more finicky than preparing QFNs in the first place. I realise this doesn't really apply to manufacturing, but automated manufacturing already has the processes to deal with QFNs.

cottonseed 13y ago
Thanks! This is super useful. I had a QFN solder failure on one of my last boards. I mistakenly tented the center vias on the bottom, forgot to clear them before reflow, and it seems hot gas escaped, popping the chip off! But it left perfectly tinned pads. I will try your method out. You reflow the whole board with a hot air station? I've been using an oven. How big a board can you do that way?
- lgeek 13y ago
  > You reflow the whole board with a hot air station? I've been using an oven. How big a board can you do that way? Yep. It's not really by choice, but because I have limited space. That being said, I like being able to watch the process and to correct minor issues like a 'tombstone' passive, so I think I'd prefer a hot plate over an oven. My SMD boards are small, the largest one by length is 7 cm and the largest one by area is 25 sq cm. You could definitely do larger ones, but you'd probably get significant temperature gradients - bad for manufacturing, probably ok for prototyping and one-offs.
- 13y ago